Motors for vehicles such as hybrid electric vehicles or electric vehicles require large torque during startup and hill-climbing. Increasing motor size is effective in increasing motor torque. However, there is a problem in doing this as it increases vehicle weight and results in reduced fuel efficiency. For this reason, such motors can be designed for use in a non-conventional, high magnetic flux density range, such as 1.9 to 2.0 T, during startup and hill-climbing.
Meanwhile, an electrical steel sheet is punched into the shape of a core constituting a rotor of a motor so that it is used as the core material. However, due to the introduction of strain associated with such punching, iron loss property will deteriorate more than before the punching. Accordingly, the resulting motor may encounter a more significant increase in motor loss than is expected for the iron loss based on its material properties. As a measure to counter such difficulties, strain relief annealing may be performed at approximately 750° C. for 2 hours. In addition, by promoting the growth of crystal grains through the strain relief annealing, a further improvement in magnetic properties can be expected. For example, JP 3458682 B discloses a technique of improving grain growth properties during strain relief annealing and reducing iron loss by increasing the amount of Al to add.
However, our investigations revealed that while strain relief annealing reduces iron loss in a conventional magnetic flux density range from about 1.0 to 1.5 T, it can rather lead to increased iron loss in a high magnetic field range. Therefore, there is a need for a technique that ensures stable reduction of iron loss in a high magnetic field. In view of the foregoing, it could be helpful to provide a non-oriented electrical steel sheet with low iron loss, particularly in a high magnetic field range.